3-phase rectifiers used frequently in ac motor drive, such as thyristor rectifiers and diode rectifiers, can pollute the ac-supply with significant levels of low-frequency harmonics and excessive VARs. With a large growing market, tougher regulations and severe economic restraints, the design of unity fundamental power factor (ufpf) 3-phase rectifiers operating with a low current total harmonic distortion (low thd) is of significant interest to many drive manufacturers and designers of power electronic equipment receiving power from 3-phase sources.
FIGS. 1A-1F, 2A-2D, 3A-3C and 4A-4D represent the prior art. The main applications for the invention are industrial variable speed drives, as shown in FIG. 1A, and any power electronic interface to a 3-phase sinusoidal supply such as the ac utility mains supply and 3-phase generators. The standard widely used low cost 3-phase diode rectifiers are shown in FIGS. 1B, 1C and 1D. The diode rectifier with a C dc filter, FIG. 1B, draws currents from the ac supply, FIG. 2A, which are rich in harmonics. The IEEE 519 standard, and IEC 555, and problems associated with high harmonics in the utility, restrict, the use of this circuit to low power. The ac supply currents associated with the rectifiers in FIGS. 1C and 1D are shown in FIGS. 2B and 2C respectively. These currents are lower in harmonics but are still not close to the 5% distortion level specified in the standards. A pulse width modulated (PWM) IGBT (Insulated Gate Bipolar Transistors) inverter bridge with a constant voltage dc rail, see FIG. 1E, and PWM GTO (Gate Turn Off Thyristor) with a constant current dc rail, see FIG. 1F, are widely used rectifiers presently being used in industry when lower line current harmonics, and regulation of the dc rail voltage or current, are required. Variations to these rectifiers, such as 12-pulse rectifiers and 3-level inverter bridges, are variations used in high power and high voltage applications. All these rectifier types produce line currents with low harmonics, as illustrated in FIG. 2D, but suffer from high electrical stresses, high per unit current ratings, filtering problems, emi/rfi emissions, high switching losses, and lower reliability.
Many circuit topologies exist that produce high performance and low line current harmonics, but with semiconductor switches that have lower electrical stresses, lower per-unit current ratings, and survivability under certain failure conditions. The rectifiers in FIGS. 3A-C are examples of circuit topologies for phase-3 pwm boost rectifiers that operate with a unity fundamental power factor and low distortion ac line current Salmon, J. C.: "Reliable 3-phase pwm boost rectifiers employing a stacked dual boost converter sub-topology", IEEE Trans. on Ind. Appl. VOL. 32, NO. 3, May/June 1996, pp. 542-551!. Overlap delays between the switching of the upper and lower devices in the pwm rectifier leg are not critical and diodes eliminate the possibility of the dc-link capacitor discharging into short circuits and shoot-through fault conditions. The rectifiers are controlled using a "stacked dual boost converter cell" sub-topology model that can be used in two current control modes. The dual current control mode shapes two line currents and can achieve current distortion levels below 5%. The single current control mode shapes one line current and can achieve current distortion levels close to 5% with the rectifier output dc voltage at the standard level associated with a rectified mains voltage. The per-unit current ratings for the switches in the 3-phase pwm switch networks are around 15-20% of the input rms line current as compared to 71% for a standard 3-phase pwm rectifier.
The standard pwm rectifier, see FIG. 1E, has the advantage of using a standard 3-phase module with a bi-directional power flow capability. This rectifier has disadvantages in terms of having a high cost, high per-unit current rating, poor immunity to shoot-through faults and high switching losses. The pwm rectifier using a boost diode, as shown in FIG. 3A, uses a standard 3-phase pwm module, hereby referred to as a pwm rectifier. The dc-rail diode provides shoot-through protection and allows the pwm rectifier to be operated using a stacked dual boost converter sub-topology model. The pwm rectifier using a 3-phase diode bridge, as shown in FIG. 3B, uses a pwm rectifier module with very low current ratings and hence has a low cost potential and a high reliability potential. All the rectifiers in FIGS. 3A-3C use a 3-phase switching network with low current ratings and can operate as a diode rectifier if the 3-phase switch module fails. The delta-connected and Y-connected bi-directional switch rectifiers, see FIG. 3C and FIGS. 4A-4D, have very low conduction losses and switching losses especially if true bi-directional switches become commercially available Salmon June 1996, above referred to and Salmon, J. C.: "Reliable 3-phase pwm boost rectifiers employing a stacked dual boost converter sub-topology", IEEE Trans. on Power Electr., VOL. 11, NO. 4, July 1996, pp. 592-603!.
FIGS. 4A-D illustrate rectifiers using Y-connected 3-phase switches with ac-line inductors. The switches S.sub.a, S.sub.b and S.sub.c are bi-directional switches and are shown in a simplified from in the figures for illustrative purposes. These switches and operation, as described in the literature Salmon, July 1996!, would be implemented using MOSFET, IGBT and GTO type switches.
A significant disadvantage of the prior art rectifiers presented in FIGS. 3A-3C and 4A-4D are their relatively convoluted and non-standard topologies, compared with a standard 3-phase rectifier, high switch count and high conduction losses. The prior art circuits would commonly be implemented using IGBT switches. The circuit switches have a relatively high peak current to rms ratio since an IGBT device is often selected based upon the peak current flowing through the device rather than the rms current, and since an IGBT switch has a relatively low peak current to rms ratio, IGBT switches are not well suited for use in these circuits.